This invention relates to optical fiber transmission systems and, more particularly, to multimode optical fibers having slight attenuation and slight dispersion.
Optical fibers comprise a central zone or core that is surrounded by a concentric layer called a cladding with a lower index of refraction. Optical signals are propagated in the core from one end of the fiber to the other by multiple total reflections or by continuous deviations at the periphery of the core.
Of particular interest to the present invention are those optical fibers known as step index optical fibers whose refractive index makes a step change at the core-cladding interface. These fibers are generally made of silica glass of very high purity and therefore show a very slight attenuation of the transmitted optical signal. Unfortunately, such fibers are the cause of a considerable deformation of said signal. Specifically, in the case of multimode fiber, an input signal will follow through the fiber different paths or modes of travel which have different lengths. As a result, in a medium with a constant refractive index, the same input signal will arrive at the output at different times spaced over a period .DELTA.t. This time dispersion limits the amount of data that can be transmitted per unit of time; and this property defines what is called the bandwidth or passband. For this type of step index fiber, the bandwidth does not exceed about ten megahertz.multidot.kilometer (MHz.multidot.Km).
To mitigate this drawback, graded index optical fibers are used whose index of refraction decreases in a quasi parabolic relationship from the axis to the periphery. In principle, this profile of the index of refraction allows signals to be propagated at different speeds depending on their path or mode of travel, which has the effect of eliminating the time dispersion and therefore providing theoretically infinite bandwidth. In practice, the dispersion is not zero but the bandwidth actually obtained is generally greater than 400 to 500 MHz.multidot.km and can even exceed a gigahertz.multidot.km (GHz.multidot.km).
This improvement in performance, however, has its economic cost: step index fibers are obtained from preforms made at relatively low cost, while parabolic profile or graded index fibers require the use of long and expensive manufacturing processes.